The man who allegedly bashed him was a prison ally, but also one of the country's most dangerous inmates, the Herald Sun newspaper reports, and is responsible for bashings and riots in the past 20 years.

It is also believed another inmate who was present during the bashing was one of Williams' lieutenants and was reportedly on the phone during the attack, giving a blow-by-blow account of the beating.

Williams was sitting at a table when he was hit from behind in an exercise facility in the Acacia Unit of the prison, southwest of Melbourne.

Williams' lawyer Rob Stary said he was gobsmacked over what happened because Williams was under constant monitoring while in solitary confinement or when released into common areas.

A prison officer was about 10m away when the bashing occurred and Williams returned to his cell before going into cardiac arrest.

Mr Stary said he was preparing an appeal to have Williams' minimum 35-year sentence reduced.

"There are other matters that are currently in the loop of great sensitivity," he told ABC Radio.

"Dare I say, he was a goldmine of information that unfortunately goes to his grave with his untimely death."

But Mr Stary said not even Williams' death would prevent allegations of corruption from reaching the public domain.

"In good time a can of worms will be revealed which I regrettably can't talk to you about now, but they were matters of great sensitivity," he said.

"So the public doesn't know in the same way as in NSW with the Wood royal commission or the Fitzgerald royal commission in Queensland the extent of corruption that existed in this state.

"And there are people who are trying to suppress it for their own self-interest."

Mr Stary has called for an independent investigation into Williams' death, with hopes of exposing those who benefited from his murder.

"I'm hopeful that there's a very thorough investigation as to all aspects of Carl Williams death — independent of Victoria police and independent of Corrections," he said.

"So that those that were going to profit from his untimely death will hopefully be fully exposed."

Williams was serving life imprisonment with a non-parole period of 35 years, after pleading guilty to ordering the murders of three underworld rivals and the failed conspiracy to murder lawyer Mario Condello.